Protective device



' NOV. 19, 1940. Q w ET AL 2,222,427

PROTECTIVE DEVICE Filed April 11, 1939 INVENTO R 5, Eric Lawrence Gas/in Wh/fe Ala/1 Dawer'B um/e/n BY 7 665441 I ATTORNEY Patented Nov. 1940 t H PROTEGTI VE DEVIQE' I t I Erie Lawrence Gasling White, 'Hillingdon, and Alan Dower Blumlein, Ealing, London England,

assignors to Electric & Musical Industries Limited, HaYGSkMiddlesexllhgland, a conipany of Y Great Britain I Application April 11, 1939, semi no; 267305 In Great BritainApril 13, 1938 The present invention relates to protective devicesfor use in connection with high voltage elec- 'trical apparatus such as cathode ray tube devices as' used for oscillograph and. television purposes.

I 'It'is the usual practice with cathode ray tube oscillographs, where-magnetic scanning (or deflection)" and/or focusing for the beam is em- 'ployed; to arrange thescanning (or deflection) coils 'or the beam focusing coil to fit closely to the outside of the envelope of the tube. The inside of the tube adjacent to the coils is usually silveredor covered with a'coating of some other conducting m'aterial which comprises, or is connected to, the final anode of the'electrongun'of 5 -the-cathode ray tube. There may be quite a large capacity through the glass betweenthe conducting coating and the coils, -for "example, 'a capacitybf the order of 0.001 microfarad', and in' additionthe potential of the coating may be, 'for example,50 ,000 volts 'po's'itive-with respect to earthi Thus ifia flash-over occurs between the final-anode and some other electrode at a lower "potential; or if 'thehigh I tension supply is short- -circuited -by I any other occurrence, the potential of the conducting :coating maysuddenly drop; to earth potential or nearly'to earth potential; I This will momentarily cause the potential of the coils,- fthrou'gh the capacitative coupling, to drop'to, say '51),000 =volts negative with respect to-earth, even though the centerpoint of thecoils or the output 1 ftrans'formerfeedihg the-scanning coils is earthed.

because the inductance of the coils preljse nts' 'a high impedance to earth for the sudden Qs'ur'gi'e. f'Such "a v potential across the coils, or part 35 of them, 3 or acros's'the output transformer, may

easily damage them and associated apparatus bemak ingit" dangerous-to handle the coils,

"etc.""'- I Itisone'of theobjects of the presentinvention ib p' roiridemeans' for substantially overcoming 12h. I H -i.';,; L ording to" the present invention' there is d;mans;forprotecting high voltage elecn a1 apparatusirom the deleterious effects aris- 45 "ing fromfa voltage-1lashov er comprising a con-' ductingfscreen positioned between an-electrode of said apparatus which is maintained at a high ivoltage and a' component of said apparatus 'ad- I *Ja' n't said' 'electrodeyso that on the occurrence I 50 ef al voltage fiashover from said ele'ctrode' only relatively small voltages are induced on said com- 1 ponent of said apparatus adjacent's'a'idl electrode. 'Ih'ere is also a -furtlier difliculty due to 'the .ifact that-the lead's' from the lowerpotential elect 555 rtrdd sfof, the' cathode -raytube to their respective static-shieldioruse in the invention. f In Fig.' ;1f isfillustratedt' a ca'thode "f2 such as is used'ior oscillographicl purposes or for the transmissi'on or reception of television *in'sidesu'rface -off the env tube 'iS indicatedate-ac may 'be'connec'ted to the'fin'alanode of the tube, or asi shown the coating may comprise the. final "40 anodei 'I'he electrode next Etc 3 is indicated at '4 and; asshown, maybe the first a'node as in the usual construction.- trode'sare connected to high potential sources,

potential sources arefusuallyzfairly long and thus have quite largeinductive reactances tosudden surges.. If. the inductances of, the leads of the lower potential electrodes are comparable with r or greater than the-inductance'of the high Volt- "55 age lead then the inductance potentiometer formed. duringa flashover causes" a large; poten-'- .tial to be applied to the lower potential electrodes 7 with consequent damage to any associated ap- .paratus.

1 "l It is also an object of the present invention to provide means for substantially overcoming thisfurther difficulty According to one feature of thefpresent invention there isprovided a capacity connected to an electrode --of the apparatuswhich is maintained at a relatively lowvoltage which capacityis large compared with-the capacity associated withthe high voltage electrode. The capacity connected to the electrode" of the apparatus whichis 'main I Ltained at a low-voltage is preferably positioned -as closei as possibleto this electrode. I In'order that thepresent invention-may be clearly understood jandreadily carried into 'efiect it-izvillnowbe described with 'reference to the drawing illustrating "diagrammatically an em "bodiment ofthe present invention in Fig. 1, and showing in Fig.2 one form of asuitable electro ray -itube pictures; '-The endwall-of' theenvelope is pro-- -vided' 'vvith a suitablescreen' 3|, such-as a fluo- I '-rescent,- mosaic or' otherscreen according'tdthe purpose for 'which the tube is designed. On the elope of the cathode'ray oiiducting coating which Normally these "two elecfor-"example,"the potential. applied to thefinahfi anode- 3" may be of the order of 50,000' volts and the potential onthe fanode d' may be about 2,000 -volts. Iniorder: to deflect the cathode ray which is produced by, a ithermionic f cathodewhich is -'conventionally illustrated inthe tube, it is neces 'B'o tsa'ry to fit suitable' coils around the tube. '7 In'the figure only one pairof defle'ctingcoils 5 is shown v.and the. coils'sfit tightly around the glass ofthe' tube. Normally. suitable: deflecting currents -are *Iappliedto the scanning coils "thus producing a 5;

varying magnetic field which suitably deflects the electron beam over the end of the tube. In the known arrangements should the final anode fiashover to the other electrodes or should the 50,000 volt supply be short-circuited for any reason, the scanning coils may momentarily take a potential of the order or 50,000 volts negative with respect to earth due to the comparatively large capacity (approximately 0.001 pf.) between the final anode 3 and the scanning coils 5. Naturally such a potential on the scanning coils is liable to damage any apparatus connected thereto, and is very dangerous to a person handling the apparatus. r

According to the present invention a means of avoiding this difficulty is to insert an electrostatic screen 5 between thecoils and the tube and. to connect the screen to earth by means of a very low impedance connection. The screen should be of metal cut into closely adjacent strips in such a Way as to prevent the formation of eddy currents by the scanning field with consequent distortion of the field and loss of deflection amplitude. For example, the screen may consist of a cylinder of metal foil cut into a number of circular strips, with a single connectingstrip parallel to the axis of the cylinder joining all the rings and extending as an earth lead. For convenience of handling the foil 43 may be cut in a fish-bone shape and may be glued to a sheet of paper M or between sheets of paper as shown in Fig. 2, in such a way that the screen can be easily wrapped around the, tube.

There is still a difficulty due to the inductance of the leads between the potential sources and the various electrodes. In practice the leads form an inductive potential divider between the very high voltage source and earth when a fiashover occurs. Should the impedance of the leads from the electrode 4 be large then the proportion of the high voltage applied to electrode 4 during a fiashover is large and damage to apparatus connected thereto, and possibly to the other electrodes and to apparatus connected to them due to voltage fiashover from the electrode 4 to these other electrodes, is inevitable.

This difiiculty is overcome according to the invention by means of the circuit arrangement shown in Fig. 1. In the first place the condensers 8 and 9 which represent respectively the capacities between the high potential terminals I0 and II and earth, are arranged such that the capacity of condenser 9 is much larger than the capacity-associated with the high voltage terminal II, represented by the condenser 8, the condenser 9 being positioned as close to the electrode 4 as possible. Thus when a fiashover occurs the proportion of potential across the condenser is small. Secondly an inductance I,

which may comprise a long solenoidal winding, is connected between the high potential terminal I0 and the final anode3. This has the effect of causing a large potential drop to occur across the inductance during a. fiashover and thus the potential applied to the other electrodes is Very much reduced. If however, the screen 6 is earthed, the comparatively large capacity between the screen 6 and the; coating 3 acts as a comparatively small impedance on the occurrence of a fiashover and thus the effect of the inductance I is mitigated. To avoid this effect an inductance I2 is connected between the screen 6 and earth. However, the connection of an inductance between the screen and earth prevents the screening action already referredto for the purpose of preventing high potentials being applied to the apparatus associated with the coils. This difficulty is overcome by winding inductance coils I3, I4, I9 and 20 in the same direction and closely coupled with inductance I2 and by connecting winding I3 in series with one end of the scanning coil 5 and winding I4 in series with the other end of the coil 5, the scanning currents being applied to terminals I5 and I6. The coils I9 and 20 are connected similarly to the other scanning coil (not shown) and are fed with scanning currents through terminals I1 and I8. The coils I2, I3, I4, I9 and 20 are wound in such a way that a surge of current through screen 6 and coil I2 maintains the potential of the scanning coil 5 the same as that of the screen 6. It will be seen therefore, that the potentials at terminals I5, I6, I! and I8 are then unaffected by a fiashover' surge. The condition for maintaining the potential of coil 5 the same as the potential of the screen 6 is thatthe inductance of coil I2 should be equal to the mutual inductance between the coil I2 and the other coils I3, I4, etc. One satisfactory way of winding the five coils I2, I3, I4, I9 and 20 is to wind a four wire (socalled quad) screened cable in a solenoid form on a long former, the-inside wires being used for coils I3, I4, I9 and 20 and the outer screening acting as coil I2. Alternatively, a double wire screened cable can be wound in solenoid form with an insulated double wire cable wound closely over the screen cable. A further construction comprises a helically wound copper strip for coil I2 with four insulated wires wound over the helix.

In the example of cathode ray tube shown the beam of electrons emanating from the oathode 31 is focused by the electrodes 33 and 35, first and second anodes 3 and 4, although it will be understood that magnetic focusing may be employed for this purpose, either in conjunction with the magnetic means shown for deflecting the beam or in cases where electrostatic defiection of the beam is employed. a

This invention is also not limited to the type of tube shown in Fig. 1 since it can be used in conjunction with any suitable type of tube Where the necessity arises. For example, it can readily be employed with a television transmitting tube in which a photoelectric image is projected on to a mosaic screen and in which the image is focused on to the screen by magnetic means. The invention is, however, especially applicable to the so-called projection type ofv cathode ray tube for reconstituting television signals in which the image is projected on to a viewing screen and in which very large operating voltages are used.

Having described our invention, what we claim is:

1. A high voltage protection system comprising a cathode ray tube having beam forming means and an anode, means to supply high voltage to said anode, deflecting coils in cooperative relationship with said anode, a conducting screen positioned between said anode and said deflecting coils, and inductor means connected to said conducting screen for allowing only relatively small voltages to be induced on said deflectingcoils upon the occurrence of a voltage fiashover from said anode.

2. A high voltage protection system comprising a cathode ray tube having beam forming means and an electrode, means to supply high 76 voltage to said electrode, deflecting coils in cooperative relationship with said electrode, a conducting screen positioned between said electrode and said deflecting coils, and inductor means connected to said conducting screen for allowing only relatively small voltages to be induced on said deflecting coils upon the occurrence of a voltage flashover from said electrode.

operative relationship with said electrode, in-

3. A high voltage protection system comprising a cathode ray tube having beam forming means and an electrode, means to supply high voltage to said electrode, deflecting coils in cooperative relationship with said electrode, inductances connected in series with said deflecting coils, a conducting screen positioned between said electrode-and said deflecting coils, and inductor means connected to said conducting screen for allowing only relatively small voltages to be induced on said deflecting coils upon the occurrence of a voltage flashover from said electrode.

4. A high voltage protection system comprising a cathode ray tube having beam forming means and an electrode, means to supply high voltage to said electrode, deflecting coils in coductances connected in series with said deflecting coils, a conducting screen positioned between said electrode and said deflecting coils, inductor means connected to said conducting screen for allowing only relatively small voltages to be induced on said deflecting coils upon the occurrence of a voltage flashover from said electrode,

and means to maintain saidhigh voltage supply and said inductor means at substantially the same potential.

5. A cathode ray tube system comprising a cathode ray tube having a cathode, a modulating v and said source at substantially the same potential.

6. A cathode ray tube system comprising a cathode ray tube having a cathode, a modulating electrode, and beam forming electrodes including at least one anode, electromagnetic coils adjacent said anode for deflecting the formed beam, an electrostatic shield interposed between said coils and said anode, an inductor connected to said shield, a source of potential connected to said anode and means to maintain said inductor and said source at substantially the same potential and inductance coils connected to said electromagnetic coils and coupled to said inductor.

7. A cathode ray tube system as claimed in claim 6 and wherein said inductance coils are coupled to each other.

8. A cathode ray tube system as claimed in claim 6 and wherein the inductance coils are coupled to each other and wherein the self-inductance of the inductor is equal to the mutual inductance between the inductor and the inductance coils.

9. A cathode ray tube system as claimed in claim 6 and wherein the inductance coils and the inductor are all wound in the same direction.

ERIC LAWRENCE CASLING WHITE. ALAN DOW'ER BLUMLEIN. 

